The present invention relates to an anti-locking hydraulic brake system. A brake system of this type is disclosed, for example, in German Published Patent Application No. 36 01 914, of which U.S. Pat. No. 4,803,840 is a counterpart.
German Published Patent Application (DE-OS) No. 36 01 914 discloses that during non-controlled decelerations, the pump is turned off, the inlet valve opened and the outlet valve closed. Pressure fluid communicates between the master cylinder and the wheel brake cylinders. The brake is applied by stepping on the pedal thereby causing pressure to build up in the master brake cylinder, propagating, through the brake conduits, into the wheel brake cylinders.
For brake skid control purposes, the pump is switched on, such that pressure fluid is transported from the reservoir into the pressure conduits. The pressure fluid is passed from the pressure conduits into both the master brake cylinder and the wheel brake cylinders. By opening and closing the inlet and outlet valves in accordance with a predetermined control pattern, the pressure in the wheel brake cylinders is varied irrespective of the pressure in the master brake cylinder. The pressure fluid that has passed into the master brake cylinder causes the working piston of the master brake cylinder to be returned to its initial position. Once it has reached that position, the central valves open to return excess pressure fluid volume to the reservoir.
The afore-described system, admittedly, has the advantage that efforts, in terms of valves, are low; however, conversely, it exhibits the disadvantage that the working piston of the master brake cylinder and, hence, the pedal, during brake skid control, is completely restored.
According to an alternative solution, German Published Patent Application (DE-OS) No. 35 27 190, of which U.S. Pat. No. 4,750,788 is a counterpart, discloses hydraulically blocking the master brake cylinder during brake skid control thereby causing the master cylinder piston to remain in the same position upon commencement of the brake skid control. The master cylinder pressure is fed as a control pressure to a control valve adjusting the pressure in the pump circuit in response to the master cylinder pressure. In this embodiment, the pedal, admittedly, is not restored, however, substantial efforts in terms of valves are required, namely an additional blocking valve and an additional control valve. Moreover, it is not reliably safeguarded such that in the event of a pump failure an adequate reserve volume is available in the master brake cylinder sufficient for emergency deceleration.